(1) What is the memory amount I can use on a PS3 with linux? 256MB or 512MB with some kind of shared memory of the graphics card?
(2) Are there benchmarks/experiences with USB sticks as swap for additional memory?

As for benchmarks, people have tried various things for swap but many of the buses for those were too slow. But to answer your question, nobody has compiled benchmarks and published them anywhere that I am aware of._________________-->
rangerpb
<--

256MB is - frankly spoken - ridiculous :/ Anyway I'll by one for our department and do some benchmarks with molecular dynamics simulations, povray and especially swapping to memory cards. More to come.
/BR